Top Ten Astronomy Stories of 2013

Another big year for astronomy made it hard to select only ten stories, but here are my choices.

1. Fireball over Russia
The event with the biggest impact - in every sense - was a 20-meter meteor over Chelyabinsk in February. A forceful reminder about detection and deflection of space rocks! This one exploded at high altitude, so when 20-30 times as much energy as the Hiroshima atomic bomb was released, the atmosphere absorbed most of it. Nonetheless the shock wave broke windows and caused other damage to thousands of buildings. About 1500 people suffered injuries needing medical treatment. Oddly, it happened on the day that asteroid 2012 DA14 was due to make a close approach. The asteroid came and went, as predicted. The two objects weren't related.

2. Chang'e and Yutu go to the Moon
Chang'e is China's lunar program. In December Chang'e-3 landed safely on the Moon and released the rover Yutu. The last soft landing on the Moon was in 1974 by the Soviet probe Luna-24. Chang'e is named for a Moon goddess in Chinese folklore and Yutu (Jade Rabbit) was her pet. In addition to those watching on Earth, the landing was observed from lunar orbit by NASA's recently-arrived LADEE (Lunar Atmosphere and Dust Environment Explorer).

3. Rare solar eclipse
There was a hybrid solar eclipse in November. It began as an annular eclipse in which the Moon is surrounded by a ring of light because it doesn't quite cover the Sun. But then it quickly changed to a total eclipse, which was also seen as a partial eclipse in many areas. Hybrid eclipses are rare. The last one occurred in 1854 and the next will be on October 17, 2172.

4. Ancient habitable environments on Mars
The evidence has been growing for a Mars that was once wet, but it doesn't mean that Mars was habitable. NASA Mars probe Curiosity's job is to search for evidence of habitability. The rover is not only a field geologist, but also a well-equipped geochemical laboratory. In March, chemical analysis of a rock sample indicated an ancient environment in which water existed for a long time. It was also neutral (neither acidic nor alkaline), not saline, and contained a number of the key chemicals of life. This doesn't prove that microbial life ever existed on Mars, but it shows that it could have.

5. Inauguration of ALMA
The Atacama Large Millimeter/submillimeter Array (ALMA, for short) was officially inaugurated in March, and is now a fully operational observatory. An array of 66 movable radio telescopes can work together in various combinations as one telescope. ALMA is designed to have ten times the resolution of the Hubble Space Telescope. Millimeter wavelengths can penetrate dust, so are used to study starbirth and planet formation. Although water vapor in the air blocks these wavelengths, ALMA won't have a problem in Chile's Atacama Desert at 5000 m (over 16,000 ft) where the air is very dry.

6. Exoplanet search comes of age
The very first exoplanets were discovered in 1992. They were orbiting a pulsar, the remains of a collapsed star. The first planets discovered orbiting sun-like stars were massive, because these are the easiest to detect. Twenty-one years later in 2013, the 1000th exoplanet was added to the Extrasolar Planets Encyclopedia. It now includes many smaller planets and multiplanet systems.

7. Earth says “cheese”
In July, two distant NASA spacecraft photographed the Earth. Astronomy groups worldwide had organized events to “wave at Saturn” as the Cassini spacecraft took its pictures. There had already been evenings of observation of Saturn. Only two other images of Earth have been taken from the outer Solar System. One was also taken by Cassini, showing Earth through Saturn's rings. The other was Voyager 1's “pale blue dot” photo. MESSENGER, too, took pictures from its Mercury orbit. However it wouldn't have been a good idea to encourage people to stare at Mercury since it's so close to the Sun.

8. Voyager 1 became a starship
Voyager 1 is the first human object to enter the space between the stars. After many previous announcements of its leaving the Solar System, in September NASA officially announced that Voyager is now bathed in the plasma of interstellar space, not the solar wind. It's still in the Solar System, as it won't pass the Oort Cloud for a few hundred years.

9. Gaia launched
Astronomers have been waiting for Gaia for a long time. It's one of the most ambitious space missions ever devised and has been two decades in the making. The European Space Agency (ESA) launched Gaia in December, and in the next five years it's expected to map positions of and distances to over a billion stars. The measurements will be done with such high precision that the error will be “equivalent to the size of a euro coin on the Moon as seen from Earth.” (A euro coin is slightly smaller than a U.S. quarter.)

10. Comet ISON
Comet ISON wasn't the comet of the century. Or even the comet of the year – there were several that fared better. Yet ISON was still unique. It was the first sun-grazer known to have come from the Oort Cloud, and it was studied extensively. Over a dozen space observatories, many large telescopes and countless amateurs provided a detailed record of its visit. This data will give astronomers valuable information about its structure, composition and how sun-grazers survive - or don't - a close passage by the Sun.